As negative working light-sensitive lithographic printing plates requiring no fountain solution in which light-sensitive layers and silicone rubber layers are successively formed on supports, various kinds of plates have been proposed.
For example, the use of compositions in light-sensitive layers have been reported in JP-A-63-88556 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-8-328240, the compositions comprising compounds generating acids by light (light acid generating agents), compounds hydrolyzed by acids to change in solubility and binder resins if necessary. JP-A-8-328240 describes a process comprising adding a compound having two or more enol ether groups or enol thioether groups and a linear polymer having an acid group and a hydroxyl or mercapto group to the above-mentioned light-sensitive layer, thereby forming a three-dimensionally crosslinked resin in coating and drying or by heating after coating and drying, hydrolyzing the crosslinked portion in the presence of an acid generated from a light acid generating agent, for an exposed area, to improve the peeling property of a silicone rubber layer at the exposed area, and effectively removing a solubilized portion of the light-sensitive layer by use of a developing solution.
On the other hand, with the recent rapid progress in prepress systems and output systems such as image setters and laser printers, methods for obtaining printing plates by converting printing images to digital data, and using new prepress processing systems such as computer-to-plate and computer-to-cylinder systems have been proposed. Accordingly, new types of printing materials for these printing systems have been desired, and the development thereof is proceeding.
JP-B-42-21879 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-50-158405, JP-A-6-55723, JP-A-6-186750, U.S. Pat. No. 5,353,705 and PCT International Publication WO-9401280 describe that ink-repellent silicone rubber layers are formed on light-heat conversion layers containing laser light absorbers such as carbon black and self-oxidizing binders such as nitrocellulose, and that the silicone rubber layers are partially removed by laser beam irradiation to form ink-receiving areas, thereby conducting waterless printing.
In the above, however, the silicone rubber layers are removed by abrasion of the light-heat conversion layers due to laser beam irradiation, and since the adhesion between the light-heat conversion layers and the silicone rubber layers is weak, JP-A-9-146265 reports a process of improving the above-mentioned adhesion by adding an addition polymer having groups reactive to a silicone crosslinking agent on side chains to a light-heat conversion layer, and allowing the polymer to react with the silicone crosslinking agent contained in a silicone rubber layer in coating and drying of the silicone rubber layer.
However, the above-mentioned known negative working light-sensitive lithographic printing plate requiring no fountain solution is insufficient yet in the adhesion between the light-sensitive layer and the silicone rubber layer, and has the problem that the plate is easily scratched in handling the printing plate and in printing, resulting in adhesion of ink on the scratched portion to form an image area. In particular, in the negative working waterless lithographic printing plate, the silicone rubber remains on an unexposed area, so that it is required that the light-sensitive layer is sufficiently adhered with the silicone rubber layer. Insufficient adhesion results in removal of the silicone rubber in a portion where the light-sensitive layer must be adhered with the silicone rubber layer in development, particularly the silicone rubber layer of a nonimage area around the periphery of an image area where the silicone rubber has been removed, which causes a reduction in resolution of an image.
Moreover, the above-mentioned process has the problem that if the crosslinking component in the light-sensitive layer is used in an increased amount for improving the reactivity with the crosslinking agent in the silicone rubber layer to improve the adhesive properties, or if the silicone rubber layer is dried at high temperature for a long period of time in coating for allowing the crosslinking reaction to occur easily, the performance of the light-sensitive layer is deteriorated to cause failure to obtain a good image.